1. Field of the Invention
The present invention relates to the field of integrated circuit assembly. More specifically, the present invention relates to a support for conveying integrated circuit elements in a soldering oven.
2. Discussion of the Related Art
After manufacturing a semiconductive integrated circuit chip, said chip must be attached on other elements before encapsulation of the assembly in a package. These elements are often attached together before assembly of the chip. Among these elements, some appear in the form of strips such as a connection frame or a heat sink, while other elements appear in the form of individualized elements such as insulating pads, for example, made of ceramic.
To attach together these different elements by batches, a soldering method is conventionally used. Such a method consists of fusing an intermediary fusible element (or solder paste) placard between two elements to be assembled. To reach the temperatures required for this fusion, each batch of elements to be soldered is placed on a support, which is exposed to a heat source in an oven.
An embodiment of a conventional soldering method is illustrated in FIG. 1.
FIG. 1 shows a transversal cross-section of an oven associated with an example of conventional conveyor support 5 and on a wall 1 of which support 5 is moved. Such an oven generally is called an on-line oven. A connection frame 2 is desired to be soldered on an isolating element 3, for example, a ceramic pad, and this same element 3 is desired to be simultaneously soldered on a heat sink 4. The assembly of these three elements rests upon support 5. A driving system (not shown) enables moving support 5 along wall 1 of the oven, for example, in the direction indicated by arrow A in FIG. 1.
To ensure a stable hold and an accurate positioning of the different elements before assembly, support 5 includes a base 5-1, typically made of aluminum. Base 5-1 has a stepped shape adapted to receiving in a low portion 5-11 heat sink strip 4 and individualized pads 3. A high portion 5-12 of the step is adapted to receiving a first strip-shaped portion 2-1 of frame 2, the lower surface of a second portion 2-2 of frame 2, forming one piece with its first portion 2-1, contacting the upper surface of each of pads 3. Between each of these elements, a determined amount of solder paste (not shown) adapted to the surfaces to be assembled, as well as to the desired thickness for the soldering between these elements, is deposited. High portion 5-12 of the support also includes a protruding portion 5-13 (formed of punctual elements or continuous), the object of which will be described hereafter.
A weight 5-2 is placed above the assembly thus obtained. Weight 5-2 has the object of holding the hold of elements 2, 3, and 4 to be soldered during the flat motion of the conveyor support, and of applying a homogeneous mechanical pressure on the surface of frame 2 to help the forming of soldered joints between frame 2 and each of pads 3 and between the pads and heat sink 4. Weight 5-2 is stabilized in a raised position with respect to surface 2-1 of frame 2, in a way which will be described hereafter, to apply an identical and homogeneous pressure on surface 2-1 of frame 2 in different punctual locations. For this purpose, weight 5-2 essentially includes a body 5-21 in which are attached two series of risers 5-22 and 5-23.
A first series of risers 5-22 counterbalances and stabilizes weight 5-2 in cooperation with protruding portion 5-13 of base 5-1 and with a bearing means (not shown) adapted to receive weight 5-2, for example, by engaging of tabs provided on each side of the lower surface of body 5-21 into reception openings (not shown) made in the vicinity of the lateral ends of base 5-1. Riser assembly 5-22 associated with the bearing means enables placing weight 5-2 in a stable and raised position with respect to frame 2. In other words, body 5-21 of weight 5-2 never is in contact with the surface of frame 2.
Weight 5-2 also includes a second series of risers 5-23 with a vertical clearance in body 5-21, each being associated with an axial stem 5-24. Stems 5-24, forming one piece with risers 5-23, can thus move along the vertical direction of FIG. 1 within the limits of the clearance of risers 5-23 in body 5-21. The lower ends of stems 5-24 bear on first portion 2-1 of connection frame 2. This contact is performed as close as possible to the step of base 5-1 as allowed by the mechanical tolerances of each of the components of support 5 (and especially the bearing means, not shown). Further, these contact points are, preferably, located at equal distance from two locations to be soldered. Thus, considering a batch in which twenty soldering operations have to be performed, that is, including a piling formed of sink strip 4, of twenty individual pads 3, and of portion 2-2 of frame 2, body 5-21 includes nineteen risers 5-23.
It should be noted that the location of the bearing means is, seen from the side, at an intermediary position between risers 5-22 and 5-23, preferably, as close as possible to risers 5-23. Of course, to ensure the stabilization counterbalancing function of weight 5-2, the weight of risers 5-22 is adapted to the distance separating the bearing means from stems 5-24 of risers 5-23.
A disadvantage of the conventional method described hereabove is the significant loss of thermal power. Indeed, the heat emitted by wall 1 of the on-line oven must propagate through base 5-1 of support 5 to reach the locations where the soldered joints have to be formed.
To minimize such losses, supports made of a material having an excellent heat conductivity, that is, aluminum, are conventionally used. However, a large thickness has to be provided for reasons of mechanical stability. Indeed, under a given minimum thickness, an aluminum support tends to deform under the effect of heat. This deformation affects the alignment of the elements to be soldered which must be accurate to one tenth of a millimeter. As a result, the benefit in terms of heat is then nullified by the thickness increase necessary to the mechanical hold.